Signal propagation conditions alter according to fading conditions. In radio paths, fading conditions can be divided into line of sight conditions (LOS) and rich scattering fading conditions (NLOS, not line of sight). From a terminal uplink performance point of view, it is desirable to achieve good cell coverage and data throughput. Operating at cell edges, especially in rural areas, can lead to a situation where an uplink connection to a base station is dropped. In some cases, objects may be located between a TX antenna and a BTS within a cell, which leads to a counterpart received signal being attenuated close to or below sensitivity level. It would therefore be desirable to provide improved TX antenna directivity towards base stations.
In 3G, HSPA, LTE and LTE CA terminals there is typically one TX (i.e. UL) antenna allowed concurrently according to 3GPP TS 36.213 chapter 8.7. Typical implementation is two RX antennas for diversity and MIMO reception.
According to 3GPP TS 36.213 chapter 8.7, if a UE transmit antenna selection is disabled or not supported by the UE the UE shall transmit from UE port 0, if closed-loop UE transmit antenna selection is enabled by higher layers the UE shall perform transmit antenna selection in response to the most recent command received via DCI Format 0, and if a UE is configured with more than one serving cell the UE may assume the same transmit antenna port value is indicated in each DCI format 0 PDCCH grant in a given subframe. If open-loop UE transmit antenna selection is enabled by higher layers, the transmit antenna to be selected by the UE is not specified.
An automotive environment is particularly challenging from a radio link performance point of view because vehicles such as cars tend to move reasonably fast in environments such as urban environments. In some cases, a vehicle cabin may attenuate radio wave propagation.
With regard to vehicles, it has been proposed to place diversity RX antennas in the side (or ‘wing’) mirrors of a vehicle, but a problem exists in how to control TX antenna selection. If a single TX antenna is placed in a fixed position in one side mirror of the vehicle, then there is an uplink problem with alternate BTSs on the other side of the vehicle. If two TX antennas are employed conveying TX uplink signals between alternate TX antennas, then there is a problem in how to select the optimal TX antenna for UL during operation.
Some prior art systems select the TX antenna according to DL signal strength, but selecting the appropriate TX antenna on the basis of RSSI can be unreliable.
In TDD radio communication systems, TX and RX are conducted at the same frequency, but in FDD, TX and RX are conducted at different frequencies which can impact on path losses. In the time domain, LOS conditions alter slowly, because there is typically a direct link between BTSs and UEs. On the other hand, NLOS radio conditions alter rapidly due to multiple reflections, for example in urban canyons. From an UL antenna selection point of view, a problem exists in that signal reflections may arrive at angles of 360 degrees around a vehicle so selection on the basis of DL RSSI is unreliable. Operating in rich scattering environments, such as urban canyons, can lead to a situation where optimal data throughput is not achieved in UL, because all information is not received at the BTS with a high enough SNR or the call may drop.
It would therefore be desirable to provide improved ways to control TX antenna selection.